Ultrasonic and megasonic energy cleaners have been in use for removing particles from immersed substrates, such as semiconductor wafers. Higher frequency megasonic cleaners were implemented as it was recognized that energy for the cleaning is limited by cavitation--the generation of bubbles in low pressure regions--and by the width of a laminar flow boundary layer. The collapse of these bubbles on the wafer surface caused damage to films on the wafer. The higher frequencies permitted the use of much higher energy before cavitation was initiated. The higher frequencies also resulted in a thinner fluid boundary layer. Both these effects permitted removal of smaller particles that more strongly adhered to the wafer surface.
However, as semiconductor wafer line widths get smaller, so does the size of particle that must be removed by wafer cleans. Typically a killer defect is created by a particle having a size that is greater than half the device line width, so emerging technologies having 0.2 .mu.m lines require that particles that are equal or greater than 0.1 .mu.m in size be removed.
Various improvements have been proposed to improve sonic cleaners, such as providing antireflection mechanisms to avoid standing waves, as described in commonly assigned U.S. Pat. No. 5,427,622. Another technique to sweep the frequency of vibration to eliminate or reduce standing waves, resonances, cavitation and non-uniform sound fields is described in U.S. Pat. No. 5,834,871. A staggered array of megasonic transducers, each emitting a focused beam of megasonic energy, and arranged so that the emitted beams from all the transducers collectively envelop the objects to be cleaned is described in U.S. Pat. No. 5,383,484. However, these ideas have not sufficiently provided the ability to remove the 0.1 micrometer and smaller particles from the surface of wafers. Thus, a better solution for megasonic cleaning is required to address the challenges of breaking fluid boundary layers and removing the smaller particles without damaging the fine lines and other films on a wafer, and solutions are provided by the following invention.